This invention relates to electric resistance heating elements, and more particularly, to thermoplastic insulated resistance heating elements and methods for their manufacture.
Electric resistance heating elements are available in many forms. A typical construction includes a pair of terminal pins brazed to the ends of a Nixe2x80x94Cr coil, which is then axially disposed through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, usually magnesium oxide. While such conventional heating elements have been the workhorse for the heating element industry for decades, there have been some widely-recognized deficiencies. For example, galvanic currents occurring between the metal sheath and any exposed metal surfaces of a hot water tank can create corrosion of the various anodic metal components of the system. The metal sheath of the heating element, which is typically copper or copper alloy, also attracts lime deposits from the water, which can lead to premature failure of the heating element. Additionally, the use of brass fittings and copper tubing has become increasingly more expensive as the price of copper has increased over the years. What""s more, metal tubular elements present limited design capabilities, since their shape can not be significantly altered without losing performance.
As an alternative to metal elements, polymeric heating elements have been designed, such as those disclosed in U.S. Pat. No. 5,586,214. The ""214 patent describes a process of making a polymeric heater in which an inner mold is used having a plurality of threads for receiving a resistance wire. The assembly is first wound with a wire and thereafter injection molded with an additional layer of thermoplastic material, which can contain a large amount of ceramic powder for improving the thermal conductivity of the device.
It has been discovered that injection molding a layer of thermoplastic material loaded with large amounts of ceramic powder can be difficult. The viscous polymeric material often fails to fill the mold details and can leave portions of resistance wire coil exposed. Additionally, there can be insufficient wetting between the over molded thermoplastic layer and the coil, with hardly any thermoplastic bonding between the inner mold and the over molded layer. This has led to failure of such elements during thermal cycling, since entrapped air and insufficient bonding create crack initiation sites. Such crack initiation sites and entrapped air also limit the heating elements"" ability to generate heat homogeneously, which tends to create hot and cold spots along the length of the element. Crack initiation sites also lead to stress cracks that can lead to shorts in emersion applications.
Efforts have been made to minimize hot and cold spots and insufficient bonding between layers of plastic materials having electrical resistance heaters disposed between their layers. In U.S. Pat. 5,389,184, for example, a pair of thermosetting composite structures are bonded together using a heating element containing a resistance heating material embedded within two layers of thermoplastic adhesive material. The two thermosetting components are permitted to cure, and then while applying pressure to the joint, electrical energy is passed through the heating element sufficient to heat the joint to above the melting temperature of the thermoplastic adhesive material. This heat fuses the layers of the thermoplastic adhesive to join the thermosetting materials together. The heating element remains within the joint after bonding and provides a mechanism to reheat the joint and reverse the bonding process in the field.
While these procedures have met with some success, there remains a need for a less expensive, and more structurally sound, electrical resistance heating element.
This invention provides a first embodiment method of preparing an electrical resistance heating element which includes the steps of providing first and second polymeric components and disposing a resistance heating material between them. The method further includes fusing the polymeric components together, preferably while applying a vacuum to minimize trapped air.
Accordingly, this invention provides, in this embodiment, a means for reducing hot spots and cold spots, as well as reducing the amount of entrapped air bubbles within polymer heating elements at a minimal cost. The improvements presented by this embodiment provide for hermetic sealing between the polymer components as well as between the electric resistance heating material and the polymer.
A lack of hermeticity has been known to be caused by the use of core element portions supported in a mold which cannot be fully encapsulated within an overlying thermoplastic material. It is also known to result from core surface geometries that never fully fuse to the over-molded plastic layer, or from relatively cool core surfaces which are not sufficiently melted by the onrush of molten polymeric material during an injection molding process.
In addition to fusing polymeric materials using their own self-contained electrical resistance heating material, this embodiment also teaches the use of pressure alone, vacuum heat treating, hot isostatic processing, sonic or friction welding, or heating within an inert gas pressure environment in order to fuse the polymer components and evacuate air from trapped crevices and seams in the heating element construction.
Additionally, the use of heating as a method of fusing and creating hermeticity, has the additional function, if applied correctly, of stress relieving polymeric components manufactured from injection or blow molding processes, for example. Injection molded parts often contain stresses at points of sharp mold impressions or corners. Such stresses are caused when the molten polymer solidifies and shrinks in the mold. This invention can employ vacuum heating, resistance heating, or both for example, to relieve the stresses in injection molded components, which creates an additional benefit for use of the final assembly as a heating element. Since many of the stresses associated with fabricating the component are reduced or eliminated by these embodiments, there are fewer defects, or crack initiation sites, which could shorten the life of the element during cyclical heating and cooling cycles.
The use of vacuum, heat and tight-fitting injection molded parts in the preferred embodiment helps to create a hermetic heating element which has fewer imperfections and a longer useful service life.
In a further embodiment of this invention, a heating element and method of construction are provided in which first and second polymeric components are joined together with a resistance heating material therebetween. The polymeric components include retention means including a plurality of male connectors located on the first polymeric component, and a plurality of female receiving recesses located on the second polymeric component for mating with the male connectors of the first polymeric component. The polymeric components can optionally be joined together by the fusing techniques of this invention.